This application claims the priority of Korean Patent Application No. 2004-101840, filed on Dec. 6, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to a method and device for controlling a motor and, more particularly, to a method and device for controlling the startup of a synchronous reluctance motor.
2. Description of Related Art
A synchronous reluctance motor implies one configured such that a driving source of a rotor is synchronous with that of a stator and the rotor rotates so that a resistance created in the rotor can be minimized when a current flows into the stator. To drive such a synchronous reluctance motor, the position of the rotor should be known. The position of the rotor can be directly detected by use of a rotor-position detector, such as an encoder. However, the rotor position detector can not be used in a compressor for a refrigerator or air-conditioner since the internal temperature of the compressor is extremely high. Thus, the synchronous reluctance motor for the compressor is controlled by means of a sensorless control scheme.
A typical sensorless control system will now be described in detail with reference to FIG. 1. The sensorless control system includes a reference torque current generator 10, a rotating-coordinate-system voltage generator 20, a stationary-coordinate-system voltage generator 30, a motor drive voltage generator 40, and a coordinate system converter 50. The reference torque current generator 10 generates a reference torque current i*q for compensating an error caused by a difference between a reference speed ω*, also called an instruction speed, and an estimated rotor speed {tilde over (ω)}. The rotating-coordinate-system voltage generator 20 generates a reference torque voltage ν*q for compensating an error caused by a difference between the reference torque current i*q and an actual torque current iq, and a reference flux voltage ν*d for compensating an error caused by a difference between a reference flux λ*d and an observed flux {circumflex over (λ)}d. The stationary-coordinate-system voltage generator 30 converts the reference torque voltage ν*q and the reference flux voltage ν*d, both of which are the rotating-coordinate-system voltages, into stationary-coordinate-system voltages ν*α,ν*β. The motor drive voltage generator 40 converts the stationary-coordinate-system voltages as ν*α,ν*β to three-phase voltages νU,νV,νW by means of a space-voltage vector pulse width modulation (SVPWM) and then applies the three-phase voltages to the synchronous reluctance motor (SynRM motor).The coordinate system converter 50 converts three-phase currents iU,iV detected upon rotation of the synchronous reluctance motor to stationary-coordinate-system currents iα,iβ and in turn converts the stationary-coordinate-system currents iα,iβ to rotating-coordinate-system currents id,iq.
In addition, the sensorless control system further includes a flux observing unit 60 for outputting an observed flux {circumflex over (λ)}αβ on α-axis and β-axis of the stationary coordinate system of the motor, an estimated flux {tilde over (λ)}dq on d-axis and q-axis of the rotating coordinate system and an observed flux {circumflex over (λ)}q on q-axis of the rotating coordinate system from stationary-coordinate-system voltages να,νβ and stationary-coordinate-system currents iα,iβ inputted to the motor.
For reference, the observed flux {circumflex over (λ)}αβ and the estimated flux {tilde over (λ)}dq are calculated from the following Equations 1 and 2 using detected voltage ναβ and currents iαβ,idq, predetermined resistance R in a stator coil, and a magnetic model.
                                          λ            ∼                    dq                =                  L          ⁡                      (                          i              dq                        )                                              [                  Equation          ⁢                                          ⁢          1                ]                                                      λ            ^                    αβ                =                                            s                              s                +                g                                      ⁢                          (                                                                    v                    αβ                                    -                                                            R                      s                                        ⁢                                          i                      αβ                                                                      s                            )                                +                                    g                              s                +                g                                      ⁢                                          λ                ∼                            αβ                                                          [                  Equation          ⁢                                          ⁢          2                ]            
In addition, the sensorless control system further includes a position/speed estimator 70 for estimating a rotating angle {tilde over (θ)} of the motor's rotor from the observed flux {circumflex over (λ)}αβ, {circumflex over (λ)}q and the estimated flux {tilde over (λ)}dq, which are outputted from the flux observing unit 60, based on a predetermined equation, and for estimating a rotor speed {tilde over (ω)} from the estimated rotating angle {tilde over (θ)}.
The sensorless control system incorporated in the synchronous reluctance motor uses a startup-through-signal-injection algorithm at an initial startup time (i.e., an interval from halt of the rotor to low speed of the rotor) when input/output information is weak. For this, a typical sensorless control system further includes a d-axis flux injector 80. The d-axis flux injector 80 is used for generating and injecting a small-sized excitation signal λ*d-inj necessary for estimating the position of the rotor.
In the sensorless control system thus configured, the observed flux {circumflex over (λ)}αβ and the estimated flux {tilde over (λ)}dq are to be obtained to estimate the position of the rotor. For this, the resistance Rs of the stator coil and the magnetic model {tilde over (λ)}dq=L(idq) are to be specified in advance. These motor constants are specified through a prearranged experiment or simulation.
Accordingly, when the specification of the synchronous reluctance motor has changed, the resistance Rs of the stator and the magnetic model should be newly obtained through an experiment or simulation. Further, there is a problem in that the obtained values are applied to a startup algorithm so that the motor should be controlled through a new startup algorithm.